Gas-operated cooking appliances usually consist of at least one and generally a greater number of burners, where the ignition of the flame of the burner is provided through use of an external spark igniter. The burner is connected to a mains gas supply line with a branch inbetween the burner and said mains gas supply. With the aid of a switch located inside a burner valve associated with the burner, ignition can be triggered individually on each burner or simultaneously on all of the burners with the aid of a centrally produced ignition signal, wherein this only results in ignition in burners whose valves are open.
If for some reason the flame disappears on one of the burners, e.g. due to a sudden air flow over the cooking appliance or a liquid poured directly on the burner, unburned gas continues to flow through the branch pipe, leading to a potentially hazardous situation on the hotplate. A known measure taken to avoid such hazardous situations is to provide each burner with an electrically controlled valve that is in operable communication with a thermocouple located in each burner and that blocks the inflow of gas to a branch pipe when the respective thermocouple notices a sudden temperature drop at the burner. A typical cooking appliance is shown by way of example in FIG. 1 where this prior art measure is shown schematically. The terms EV1 to EV4 in FIG. 1 stand for electrically controlled valves used to block gas flow into the branch pipes for burners B1 to B4 respectively. T1 to T4 refer to the thermocouples located in the vicinity of the burners B1 to B4 whereas KL indicates the mains gas supply line.
It is known in the prior art that attention should be paid to child safety when reestablishing a flame on a burner whose flame previously extinguished. The example shown in FIG. 1 can be re-operated by mechanically actuating electrically operated valves (EV1-EV4) and ignition can be provided. Due to the fact that burners (B1-B4) cook independently from each other, all other burners that are in operation and that keep their flame burning remain burning, in cases where the flame extinguishes in one of the burners. This clearly creates a new risk in terms of system safety. A second drawback of this solution emerges from the fact that gas-operated cooking appliances generally have more than one burner (mostly four) and it is a very expensive solution to equip each and every burner with an electrically controlled valve for blocking gas flow.
EP 0 727 616 A1 discloses an improved safety device for eliminating the afore-cited safety risks. The system in EP 0 727 616 A1 closes the gas flow into the mains gas supply line instead of into branch lines, as a result of which the cooking appliance is clearly cheaper. According to the teaching of EP 0 727 616 A1, provision is made for a childproof lock in order to avoid unwanted re-lighting of a flame on an extinguished burner. The electromagnetic valve continues to block gas flow from the mains even if an individual burner valve is opened. In order to reactivate the system it is necessary to simultaneously open or close two gas taps or a separate pushbutton shall be pressed for a certain duration. In spite of the fact that additional safety measures have been described in EP 0 727 616 A1, it is known that these measures are not efficient in preventing children from inappropriately activating the blocked safety system of the cooking appliance. A child playing with the taps may open or close any two taps simultaneously or press a pushbutton long enough to activate the system. The safety system proposed in EP 0 727 616 A1 is not perfectly safe due to the play behavior of children and may randomly or unfortunately fail to operate correctly if children play with the valves of this cooking appliance.